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Cohen-Lenstra sums over local rings

Christian Wittmann (2004)

Journal de Théorie des Nombres de Bordeaux

We study series of the form M | Aut R ( M ) | - 1 | M | - u , where R is a commutative local ring, u is a non-negative integer, and the summation extends over all finite R -modules M , up to isomorphism. This problem is motivated by Cohen-Lenstra heuristics on class groups of number fields, where sums of this kind occur. If R has additional properties, we will relate the above sum to a limit of zeta functions of the free modules R n , where these zeta functions count R -submodules of finite index in R n . In particular we will show that...

Copure injective resolutions, flat resolvents and dimensions

Edgar E. Enochs, Jenda M. G. Overtoun (1993)

Commentationes Mathematicae Universitatis Carolinae

In this paper, we show the existence of copure injective preenvelopes over noetherian rings and copure flat preenvelopes over commutative artinian rings. We use this to characterize n -Gorenstein rings. As a consequence, if the full subcategory of strongly copure injective (respectively flat) modules over a left and right noetherian ring R has cokernels (respectively kernels), then R is 2 -Gorenstein.

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